The present invention relates to a technique of converting tone image data into image data expressed by the on-off state of various dots.
An image display device that creates dots on a display medium, such as a printing medium or a liquid crystal display screen, to express an image is widely used as the output device of diverse imaging equipment. The image display device is capable of expressing only either the dot on state or the dot off state in each pixel. Adequate regulation of the dot creation densities according to the tone values of the image, however, enables expression of a multi-tone image.
In order to ensure creation of dots at the adequate densities according to the tone values of the image, the error diffusion method is widely applied to specify the dot on-off state in each pixel. The error diffusion method diffuses an error of tone expression, which arises due to creation or non-creation of a dot in each pixel of interest, into non-processed pixels that are in a neighborhood of the pixel of interest, and specifies the dot on-off state in each non-processed pixel to cancel the sum of error divisions diffused from its peripheral pixels. The error diffusion method specifies the dot on-off state to cancel the sum of error divisions and thus ensures creation of dots at the adequate densities according to the tone values of the image. Application of the error diffusion method thus enables high-quality images to be displayed by the image display device.
As the recent trend, image display devices that are capable of actively regulating the tone values expressed by the respective dots have been used widely. The tone values expressed by the respective single dots are regulated, for example, by using variable size dots or by selectively using inks of different densities for dot creation. In the image display device, regulation of the tone values expressed by the respective dots according to the tone values of the image desirably reduces the occurrence of significant errors. The error diffusion method may be applied for specification of the dot on-off state. This technique creates dots of adequate tone values according to the error divisions diffused from peripheral pixels and thereby quickly cancels the errors. In the image display device that is capable of creating variable dots having different tone values to be expressed, application of the error diffusion method for specification of the dot on-off state ensures display of images having high picture quality.
When the variable dots having different tone values to be expressed are used for printing, however, the greater number of the variable dots makes the processing more complicated. It is accordingly difficult to quickly convert the image data into data of the specific expression format based on the dot on-off state.
The object of the present invention is thus to solve the drawbacks of the prior art technique and to provide a technique of quickly converting image data into data of a specific expression format based on the on-off state of variable dots, while maintaining the sufficiently high picture quality.
At least part of the above and other related objects is attained by an image processing apparatus that converts image data representing a tone value of each pixel into intermediate data representing dot creation densities of variable dots, that is, a first dot, a second dot, and a third dot, which have different tone values expressed by respective single dots, and determines on-off state of each of the variable dots in each pixel based on the converted intermediate data, thereby converting the image data into dot data expressed by the on-off state of the each variable dot. The image processing apparatus includes: a first intermediate data conversion module that converts the image data into first intermediate data representing a sum of dot creation density of the first dot and dot creation density of the third dot among the variable dots; a second intermediate data conversion module that converts the image data into second intermediate data representing a sum of dot creation density of the second dot and dot creation density of the third dot among the variable dots; a first dot creation specification module that specifies the on-off state of the first dot in each pixel, based on the first intermediate data; a second dot creation specification module that specifies the on-off state of the second dot in each pixel, based on the second intermediate data; and a third dot creation specification module that specifies creation of the third dot in place of the first dot and the second dot, in a pixel where both the first dot and the second dot are specified to be in the on state.
There is an image processing method corresponding to the above image processing apparatus. The present invention is accordingly directed to an image processing method that converts image data representing a tone value of each pixel into intermediate data representing dot creation densities of variable dots, that is, a first dot, a second dot, and a third dot, which have different tone values expressed by respective single dots, and determines on-off state of each of the variable dots in each pixel based on the converted intermediate data, thereby converting the image data into dot data expressed by the on-off state of the each variable dot. The image processing method includes the steps of: converting the image data into first intermediate data representing a sum of dot creation density of the first dot and dot creation density of the third dot among the variable dots; converting the image data into second intermediate data representing a sum of dot creation density of the second dot and dot creation density of the third dot among the variable dots; specifying the on-off state of the first dot in each pixel, based on the first intermediate data; specifying the on-off state of the second dot in each pixel, based on the second intermediate data; and specifying creation of the third dot in place of the first dot and the second dot, in a pixel where both the first dot and the second dot are specified to be in the on state.
The technique of the image processing apparatus and the corresponding image processing method converts the image data into the first intermediate data and the second intermediate data, and specifies the on-off state of the first dot in each pixel based on the first intermediate data and the on-off state of the second dot in each pixel based on the second intermediate data. The technique then specifies creation of the third dot in place of the first dot and the second dot in the pixels where the first dot and the second dot are to be created in an overlapping manner. The first intermediate data and the second intermediate data are set by taking into account such replacement with another dot.
This simple technique quickly specifies the on-off state of the third dot and thereby enhances the speed of the whole processing for converting the image data into data of a specific expression format based on the on-off state of the variable dots.
In the image processing apparatus, the error diffusion method may be applied to specify the on-off state of the first dot and the second dot. The error diffusion method diffuses a tone error, which arises in each pixel of interest due to specification of the dot on-off state in the pixel, into peripheral pixels and specifies the dot on-off state in each subsequent pixel of interest to cancel the sum of error divisions diffused to the pixel, thereby ensuring a resulting image of high picture quality. The diffusion of the tone errors, however, takes some time. The preferable procedure of the present invention applies the error diffusion method for specification of the on-off state of the first dot and the second dot so as to enhance the picture quality, while specifying creation of the third dot in the pixels where the first dot and the second dot are to be created in an overlapping manner. This arrangement desirably attains both the improved picture quality and the enhanced processing speed.
In the image processing apparatus where the error diffusion method is applied to specify the on-off state of the first dot and the second dot, the third dot may be a most inconspicuous dot among the variable dots.
The third dot is created in the pixels where the first dot and the second dot are to be created in an overlapping manner by coincidence. The factor of probability is involved in creation of the third dot, so that it is difficult to regulate creation of the third dot, compared with the first dot and the second dot whose on-off state is directly specified by the error diffusion method. Even if the third dot is possibly created in a non-optimum state, the arrangement of setting the most inconspicuous dot among the variable dots to the third dot preferably minimizes the potential adverse effects on the picture quality.
In one preferable embodiment, the image processing method converts the image data into dot data of a specific expression format based on the on-off state of variable size dots, and the third dot is a largest size dot among the variable size dots.
The largest size dot among the variable size dots starts creation after the other variable size dots have been created at relatively high densities. Creation of the large size dot under such conditions does not make the large size not significantly recognizable. The largest size dot is accordingly the most inconspicuous dot among the variable size dots. Even if the third dot is possibly created in a non-optimum state, the arrangement of setting the largest size dot to the third dot preferably minimizes the potential adverse effects on the picture quality and enables the image data to be quickly converted without worsening the picture quality.
In one preferable application of the image processing apparatus, it is determined whether or not each pixel of interest is unsuitable for creation of the third dot, based on a tone value of the pixel. When one of the first dot and the second dot is specified to be in the on state in the pixel unsuitable for creation of the third dot, creation of the other of the first dot and the second dot is restricted in the pixel.
The application reduces the chance of creating the first dot and the second dot in an overlapping manner, when the pixel of interest is not suitable for creation of the third dot. This arrangement accordingly restricts creation of the third dot in the pixel of interest, thus maintaining the high picture quality.
There is another procedure alternative to the above application that, when one of the first dot and the second dot is to be created in the pixel unsuitable for creation of the third dot, restricts creation of the other of the first dot and the second dot in the pixel. The alternative procedure generally restricts creation of the other of the first dot and the second dot in each pixel of interest when one of the first dot and the second dot has been specified to be in the on state in the pixel of interest, while allowing creation of the other of the first dot and the second dot in the pixel of interest that is not unsuitable for creation of the third dot.
In the image processing apparatus of the above application, when the pixel of interest is not suitable for creation of the third dot, one preferable arrangement changes a threshold value used for specification of the dot on-off state in the error diffusion method, thus restricting creation of the second dot or the first dot.
In another preferable application of the image processing apparatus, it is determined whether or not each pixel of interest is unsuitable for creation of the third dot, based on a tone value of the pixel. When one of the first dot and the second dot is specified to be in the on state in the pixel unsuitable for creation of the third dot, creation of the other of the first dot and the second dot is prohibited in the pixel.
The application prevents the first dot and the second dot from being created in an overlapping manner, when the pixel of interest is not suitable for creation of the third dot. This arrangement accordingly prohibits creation of the third dot in the pixel of interest, thus maintaining the high picture quality.
The image processing apparatus of the present invention may have the following construction, when the image data of each color is converted into dot data expressed by the on-off state of variable dots having different sizes.
The present invention is thus directed to another image processing apparatus that converts image data representing a tone value of each pixel with regard to each of multiple colors into intermediate data representing dot creation densities of variable dots, that is, a first dot, a second dot, and a third dot, which have different tone values expressed by respective single dots, and determines on-off state of each of the variable dots in each pixel based on the converted intermediate data, thereby converting the image data into dot data expressed by the on-off state of the each variable dot with regard to each of the multiple colors. The image processing apparatus includes: a first image data conversion module that converts the image data of a first color into the dot data with regard to each of the multiple colors, where the first color is selected in advance among the multiple colors such that conspicuousness of a dot having a largest tone value expressible for each color is in an allowable range when an image is formed according to the dot data; and a second image data conversion module that converts the image data of a second color into the dot data with regard to each of the multiple colors, where the second color is selected among the multiple colors except the first color. The first image data conversion module has: a first intermediate data conversion sub-module that converts the image data of the first color into first intermediate data representing a sum of dot creation density of the first dot and dot creation density of the third dot among the variable dots, where the first dot has a smallest tone value expressed among the variable dots and the third dot has a largest tone value expressed among the variable dots; a second intermediate data conversion sub-module that converts the image data of the first color into second intermediate data representing a sum of dot creation density of the second dot and dot creation density of the third dot among the variable dots; a first dot creation specification sub-module that specifies the on-off state of the first dot in each pixel, based on the first intermediate data; a second dot creation specification sub-module that specifies the on-off state of the second dot in each pixel, based on the second intermediate data; and a third dot creation specification sub-module that specifies creation of the third dot in place of the first dot and the second dot, in a pixel where both the first dot and the second dot are specified to be in the on state. The second image data conversion module has: a variable dot intermediate data conversion sub-module that converts the image data of the second color into intermediate data representing a dot creation density of each of the variable dots or the first through the third dots; and a variable dot creation specification sub-module that specifies on-off state of each of the variable dots in each pixel, based on the intermediate data.
This image processing apparatus is provided with the first image data conversion module and the second image data conversion module. The first image data conversion module converts the image data into the first intermediate data and the second intermediate data and specifies the on-off state of the first dot and the second dot based on the first and the second intermediate data. In the pixel where the first dot and the second dot are to be created in an overlapping manner, the first image data conversion module specifies creation of the third dot in place of the first dot and the second dot. This series of processing eventually converts the image data into the dot data with regard to the first dot, the second dot, and the third dot. The second image data conversion module, on the other hand, converts the image data into the intermediate data with regard to the variable dots, the first through the third dots, and specifies the on-off state of the first through the third dots based on the intermediate data.
The first image data conversion module is used for conversion of the image data into the dot data with regard to the first color selected among the multiple colors as the specific color that makes the conspicuousness of the dots having the largest tone values to be expressed in a predetermined allowable range in a resulting image generated according to the dot data. The second image data conversion module is used, on the other hand, for conversion of the image data into the dot data with regard to the second color, which is selected among the multiple colors except the first color.
In this arrangement, with regard to the first color that makes even the dots having the largest tone values to be expressed significantly inconspicuous, the first image data conversion module is used to quickly convert the image data into the dot data. The second image data conversion module is applied for conversion of the other image data into the dot data. Unlike the first image data conversion module, the second image data conversion module specifies the dot on-off state based on the intermediate data even with regard to the dots having large tone values to be expressed, thus assuring adequate distribution of the dots having large tone values to be expressed. The cooperative use of the first image data conversion module and the second image data conversion module preferably enables the image data to be quickly and adequately converted into dot data.
In accordance with one preferable application of the image processing apparatus provided with the first image data conversion module and the second image data conversion module, the first image data conversion module converts the image data of at least one of cyan, magenta, yellow, and black, whereas the second image data conversion module converts the image data of at least one of light cyan, light magenta, and dark yellow.
Cyan, magenta, and black dots are relatively inconspicuous, since the dots having large tone values to be expressed with regard to these colors are created in darker images (having lower lightness). Yellow is an originally inconspicuous color, so that yellow dots having even large tone values to be expressed are not visually recognizable. The first image data conversion module is accordingly used for conversion of the image data of these colors and enables the image data to be quickly converted into dot data that ensures inconspicuousness of dots in a resulting image. Light cyan, light magenta, and dark yellow dots are, on the other hand, relatively conspicuous, since the dots having large tone values to be expressed with regard to these colors are created in lighter images (having higher lightness). The second image data conversion module is accordingly used for conversion of the image data of these colors and enables the image data to be converted into adequate dot data that ensures inconspicuousness of dots in a resulting image.
The image processing apparatus of the present invention is preferably applicable to a print control apparatus that outputs dot data representing controlled creation of dots to a printing device, which creates ink dots to print an image on a printing medium, and thereby controls the printing device. The image processing apparatus discussed above enables the image data to be quickly converted into dot data expressed by the on-off state of variable dots, while maintaining the sufficiently high picture quality. Application of the image processing apparatus to the print control apparatus thus preferably ensures high-speed printing of high-quality images.
The technique of the present invention may be attained by a computer program that is read and executed by the computer to actualize the image processing method discussed above. Namely another application of the present invention is a recording medium corresponding to the image processing method.
The present invention is accordingly directed to a recording medium in which a computer program is recorded. The computer program actualizes a method that converts image data representing a tone value of each pixel into intermediate data representing dot creation densities of variable dots, that is, a first dot, a second dot, and a third dot, which have different tone values expressed by respective single dots, and determines on-off state of each of the variable dots in each pixel based on the converted intermediate data, thereby converting the image data into dot data expressed by the on-off state of the each variable dot. The computer program causes a computer to attain the functions of: converting the image data into first intermediate data representing a sum of dot creation density of the first dot and dot creation density of the third dot among the variable dots; converting the image data into second intermediate data representing a sum of dot creation density of the second dot and dot creation density of the third dot among the variable dots; specifying the on-off state of the first dot in each pixel, based on the first intermediate data; specifying the on-off state of the second dot in each pixel, based on the second intermediate data; and specifying creation of the third dot in place of the first dot and the second dot, in a pixel where both the first dot and the second dot are specified to be in the on state.
The computer reads the program recorded in this recording medium to attain the respective functions discussed above. This enables the image data to be quickly converted into the specific expression format based on the dot on-off state, while maintaining the sufficiently high picture quality.